Electrical Characterization of NCP- and NCF-Bonded Fine-Pitch Flip-Chip-on-Flexible Packages

Electronic portable devices are aimed towards higher response speed with a better viewing resolution display. Nonconductive paste (NCP) and nonconductive film (NCF) are the adhesive materials used in fine-pitch display applications. This study compares two commercially available adhesives for fine-pitch chip-on-flex (COF) applications. The electrical performance of the NCP-bonded COF was better compared to the NCF. The rheological properties of these materials in the initial stages and the mechanical properties of the adhesives after bonding are claimed to be the main factors. The semisolid form of the NCF which melts and flows slowly from the interconnection joints finally reduced the effective contact area in the joint as compared to the NCP. A low-pressure bonding caused entrapment of adhesive in the joints, induced stress accumulation in the Z-direction during high thermal loading, and a high coefficient of thermal expansion (CTE) mismatch in between bumps, adhesive, and electrode traces on the flexible substrate were the key factors for the degradation of electrical conductivity. A high load of 100 N and above was recommended since the effective contact area built into the interconnection was good and reliable after 400 cycles of a thermal shock test of -55 °C-125 °C. The NCP with a higher elastic modulus which ensures higher stiffness and stability towards elongation gave a better reliability in this environmental test. Cross sectioning and SEM analysis provide evidence of the effective contact area of the joint before and after the thermal cycle environmental test.